diff --git a/scripts/diffex-test.ps1 b/scripts/diffex-test.ps1
index b4c2f5f04..0ce1e05f1 100644
--- a/scripts/diffex-test.ps1
+++ b/scripts/diffex-test.ps1
@@ -59,13 +59,14 @@ if ($noEnv) {
 	Write-Host "Running without API keys..."
 }
 
-$tsxBin = Join-Path $repoRoot "node_modules/.bin/tsx.cmd"
-if (-not (Test-Path -LiteralPath $tsxBin)) {
-		throw "tsx not found at $tsxBin. Run npm install from the repository root first."
+$tsxLoader = Join-Path $repoRoot "node_modules/tsx/dist/loader.mjs"
+if (-not (Test-Path -LiteralPath $tsxLoader)) {
+		throw "tsx not found at $tsxLoader. Run npm install from the repository root first."
 }
 
 $cliPath = Join-Path $repoRoot "packages/coding-agent/src/cli.ts"
-& $tsxBin $cliPath @forwardArgs
+$env:TSX_TSCONFIG_PATH = Join-Path $repoRoot "tsconfig.json"
+& node --import $tsxLoader $cliPath @forwardArgs
 $exitCode = $LASTEXITCODE
 if ($exitCode -ne 0) {
 	exit $exitCode
diff --git a/scripts/diffex-test.sh b/scripts/diffex-test.sh
index b08768775..064f2d42b 100755
--- a/scripts/diffex-test.sh
+++ b/scripts/diffex-test.sh
@@ -64,4 +64,5 @@ if [[ "$NO_ENV" == "true" ]]; then
   echo "Running without API keys..."
 fi
 
-exec "$REPO_ROOT/node_modules/.bin/tsx" --tsconfig "$REPO_ROOT/tsconfig.json" "$REPO_ROOT/packages/coding-agent/src/cli.ts" ${ARGS[@]+"${ARGS[@]}"}
+export TSX_TSCONFIG_PATH="$REPO_ROOT/tsconfig.json"
+exec node --import "$REPO_ROOT/node_modules/tsx/dist/loader.mjs" "$REPO_ROOT/packages/coding-agent/src/cli.ts" ${ARGS[@]+"${ARGS[@]}"}
diff --git a/scripts/diffex-test.test.mjs b/scripts/diffex-test.test.mjs
new file mode 100644
index 000000000..8e0657e75
--- /dev/null
+++ b/scripts/diffex-test.test.mjs
@@ -0,0 +1,24 @@
+import assert from "node:assert/strict";
+import { spawnSync } from "node:child_process";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { fileURLToPath } from "node:url";
+import test from "node:test";
+
+const launcher = fileURLToPath(new URL("./diffex-test.sh", import.meta.url));
+
+test("source launcher runs from another directory and forwards arguments", (t) => {
+	const root = mkdtempSync(join(tmpdir(), "diffex-launcher-test-"));
+	t.after(() => rmSync(root, { recursive: true, force: true }));
+	const result = spawnSync(launcher, ["--no-env", "--help"], {
+		cwd: root,
+		env: { ...process.env, DIFFEX_CODING_AGENT_DIR: join(root, "agent"), DIFFEX_OFFLINE: "1" },
+		encoding: "utf8",
+		timeout: 30_000,
+	});
+	assert.ifError(result.error);
+	assert.equal(result.status, 0, result.stderr);
+	assert.match(result.stdout, /^Running without API keys\.\.\.\nDiffex - AI coding assistant/m);
+	assert.match(result.stdout, /Usage:\n  diffex \[options\]/);
+});